1. Field of the Invention
The present invention relates to a device for the transmission of light by means of optical conductors between a first and second location and particularly useful in either an apparatus for representing symbols on a display surface or an optical display apparatus for railroad and street traffic control devices.
2. Prior Art
Optical conductors have been used for projecting and illuminating purposes. These optical conductors usually consist of flexible light guiding fibers in which light entering one end of the fiber or element is conducted to the opposite end by multiple total internal reflection along the exterior surface of the fiber. Since these optical fibers are usually extremely thin and only have a thickness of a few .mu. , they are usually gathered together in bundles of approximately 500 fibers or elements and provided with an outer coating or jacket which acts as a tube or jacket for protecting the fibers or elements against mechanical influences.
If an image is projected at one end of a group of conducting elements or fibers and is to be displayed on a surface at the other end, each of the conducting fibers or elements must be maintained in a known position with the position of the output end of each fiber corresponding to the position of the input end. However, when a conductor bundle of fibers or elements is used for pure and simple illumination, the optical fibers or elements within a bundle may be unordered and the positions of the ends of each fiber or element may assume any arbitrary position. Thus, the use of one or more conductor bundles in a pure and simple illumination application enables construction of the bundles at a substantially lower cost than when the fibers are used for transmitting an image.
German Gebrauchsmuster No. 1,952,072 discloses an example of a device or arrangement for illuminating symbols utilizing a plurality of conductor bundles which contains one or more conducting elements. As illustrated in FIG. 1 of this example of prior art, an image, such as a number, is composed of a plurality of pictures or image points which are illuminated in accordance to need. To illuminate each of the points, a light guiding cable or bundle of light conducting elements or fibers is provided for each point and extends from a source of illumination to the respective individual image point on the display surface.
In use of such a prior art device, it has been shown that the intensity of illumination at the individual raster or picture points of the image is different from point to point. The intensity fluctuations increase as the number of conductor bundles proceeding from each illuminating body or source increases. It is believed that the reason for the fluctuations in the light intensity is that the individual conductor bundles are gathered together into a conductor strand or cable adjacent the source of illumination and form a closely compact surface element which is the light entry surface of the conductor strand. The intensity of illumination for each of the individual raster points is then dependent on the light distribution across the light entry surface which is formed by the ends of the gathered together conductor bundles. This light distribution is dependent on the construction of the illuminous source and on the composition and quality of any reflectors or condenser lenses which are used for projecting the light from the source onto the entry surface.
It has been suggested that the disadvantage of a nonuniform illumination of individual image points could be avoided by arranging the optical conductors or fibers in a uniform distribution at the end at which they are brought together in the conductor bundle or where several conductor bundles are gathered into a conductor strand. However, while such arrangements are theoretically possible, they involve expensive manufacturing steps and thus prevent the economical use of the arrangement.